The aim of this article is the investigation of the effects of tool tilt angle on the friction stir welding of aluminium to the steel butt joint. For this purpose, 1°, 2° and 3° tilt angles are selected to friction stir welding of AA1100 to A441 AISI, while the other process parameters (i.e. tool rotational speed, travelling speed, tool offset and plunge depth) kept constant. The results showed that with increasing tool tilt angle, the interaction between two metals and axial force increased. The increasing tool tilt angle caused more surfaces to mingle, internal mixing and frictional heat generation. The results of the microstructure of joints revealed that the AA1100 microstructure is more thermo-mechanically affected than A441 AISI. The AA1100 average grain sizes at stir zone were 1.2, 1.6 and 2 µm and at A441 AISI were 6, 7 and 9 µm at 1°, 2° and 3° tilt angles, respectively. The maximum tensile strength of joints was 75% of the aluminium base metal, which was produced at 2° tilt angle. The higher heat generation and axial force at upper tilt angle cause separation of the steel fragments on the aluminium matrix and formation of Al-Fe intermetallic compound. These phenomena lead to increase in the micro-hardness of the joint at the upper tool tilt angle.